1. Field of the Invention
This invention relates to methods for the formation of a mat of directionally oriented particles of lignocellulosic material prior to pressing of such a mat to form reconstituted pressed comminuted products.
2. Prior Art Relating to the Disclosure
Directionally oriented products of reconstituted lignocellulosic materials are desirable from the standpoint of using such reconstituted products for structural purposes. Previously, uses of such reconstituted products were limited largely to those where structural considerations were not necessary, as in floor underlayment and furniture cores.
The structural properties of consolidated lignocellulosic material products made from directionally oriented fibers or flakes are conveniently measured in terms of their "orientation index" or O.I., which is simply a numerical quantity indicating the degree of preferential alignment of the lignocellulosic material making up the product. The "orientation index" is defined as the modulus of elasticity in the oriented direction (X) divided by the modulus of elasticity in the cross oriented direction (Y) or: EQU O.I=MOE.sub.X /MOE.sub.Y
The orientation index of a reconstituted lignocellulosic material product is dependent on a number of factors, including the type of lignocellulosic material from which it is made, the density of the pressed product and the method of orientation.
The production of directionally oriented products from lignocellulosic materials such as wood fiber, flakes and/or particles using mechanical orientation of the lignocellulosic material prior to consolidation of the mat of fibers is known, and equipment for doing so is commercially available. Recently a considerable amount of research has been carried out to develop a commercially feasible method and system for electrostatically orienting discrete pieces of lignocellulosic material during formation of a mat of such material and prior to consolidation of the mat under heat and pressure.
U.S. Pat. Nos. 3,843,756 and 3,954,364 describe a method and apparatus for electrostatically orienting discrete pieces of lignocellulosic material, both on a batch and continuous basis. Products produced by the continuous process described in the above patents have not been commercially acceptable due to distortion of electrostatic lines of force in the orienting zone between the spaced charged plates immediately above the mat support surface on which the oriented fibers are deposited. This distortion of the lines of force causes the pieces of lignocellulosic material, earlier directionally oriented by the electric field established between the spaced electrodes plates, to realign themselves with the distorted directional electric field existing immediately above the mat support surface.
Methods to improve the orientation index in the production of directionally oriented mats of pieces of lignocellulosic material are described in U.S. Pat. Nos. 4,111,294 and 4,113,812. U.S. Pat. No. 4,111,294 describes the use of flexible, controlled resistive material secured to the lower ends of each of the spaced planar electrodes and extending to a region adjacent the mat being formed to maintain the lines of force of the directional electric field substantially horizontal from the top of the spaced electrode plates to a region adjacent the mat being formed. U.S. Pat. No. 4,113,812 utilizes means to force an electrical current to flow within the mat being formed to provide a directionally electric field immediately above the mat being formed parallel to the direction of movement of the mat support surface and the directional electric field in the orienting zone formed between the spaced planar electrodes above the mat support surface. Various means are described in the patent for causing an electrical current to flow within the mat between the spaced electrodes such as (1) electrodes which contact the top surface of the mat at uniformly spaced intervals, (2) electrodes on the mat support surface contacting the bottom surface of the mat, and (3) electrically conductive finger electrodes secured to the mat support and extending upwardly into the mat and downwardly through the mat support surface.
German patent publication (Offenlegungsschrift) No. 2,405,995 describes a process and apparatus for aligning fiber material in the production of compression-molded parts. The fibers in the mold are subjected to vibratory motion directed transversely of the load lines in the molded piece or held in suspension by an air stream so that the fibers are aligned in the direction of the load lines. Simultaneously the fibers are also subjected to an electrostatic field whose lines of force are aligned parallel to the load lines of the molded piece.
Swedish patent publication (Utlaggningsschrift) No. 400 223 describes a batch process of overcoming the problem of distortion of the electrostatic lines of force by using spaced electrode plates having fingers on their lower ends which project down into the mat of electrostatically oriented fibers being deposited. The electrode plates are raised as the thickness of the mat of fibers being deposited increases to prevent formation of localized weak points in the formed mat.